1. Technical Field
The present invention relates to an apparatus and process for solid-state deposition and consolidation of high velocity powder particles entrained in a subsonic or sonic gas jet onto a substrate material. Upon impact the powder particles undergo plastic deformation which permits adhesive bonding to the substrate and inter-particle metallurgical bonding. This adhesive and cohesive bonding permits coatings of substrates and spray forming of near net shape components and parts. The basic embodiment of the invention uses a friction-compensated sonic nozzle to accelerate powder particles to high velocities with several methods for heating (thermal-plastic conditioning) the powder particles and substrate to temperatures sufficiently high to reduce the yield strength during impact and permit plastic deformation at low flow stress levels. One method of the heating the powder particles and substrate uses an ambient pressure thermal-transfer plasma between the nozzle exit and the substrate. A complementary embodiment of the invention uses a powder reactor to alter the physical, chemical, or nuclear properties of powder particles prior to injection into a friction-compensated sonic nozzle for acceleration.
The solid-state deposition and consolidation process of the invention relates to a method for thermal-plastic conditioning or heating of the powder particles and substrate materials to reduce their yield strengths and permit plastic deformation at low flow stress levels during high velocity impact. This is accomplished at temperatures well below the melting points of said powder particles and substrate materials.
2. Background Art
The coating applicator and process disclosed in U.S. Pat. No. 5,795,626 issued to Gabel and Tapphorn has a low deposition efficiency, which is attributed to the high elastic response of triboelectrically charged powder particles at ambient temperature that have not been thermal plastically conditioned to induce plastic deformations. This elastic response tends to mechanically reflect the majority of impacting particle, which precludes significant adhesion or cohesion.
This is particularly true for large diameter particles, hard substrates, or work hardened depositions and substrates. Thus, the coating applicator and process disclosed in U.S. Pat. No. 5,795,626 is not economically viable for commercial applications without thermal plastically conditioning the powder particles to induce plastic deformations. Limitations to the prior art were overcome in U.S. Pat. No. 6,074,135 issued to Tapphorn and Gabel, which disclosed various methods for fluidizing and treating powder particles at high carrier gas pressures prior to injection into a supersonic applicator. U.S. Pat. Nos. 5,795,626 and 6,074,135 both describe a coating or ablation applicator that uses supersonic nozzles to accelerate triboelectrically charged powder particles in a supersonic carrier gas. Supersonic nozzles, however are extremely inefficient for accelerating powder particles to high speeds because the flow expansion process for achieving high supersonic gas speeds inherently decreases the drag force on the powder particles. The reduction in drag force is due to the precipitous decrease in gas density that accompanies the supersonic acceleration of the gas during expansion. Thus, the new art of this invention is required to enhance the solid-state consolidation processes to make it more economically attractive for commercial applications while minimizing in-situ oxidation and unwanted chemically reactivity during the deposition.
Thermal spray, plasma spray, and detonation coating methods (e.g., U.S. Pat. No. 2,714,563 issued to Poorman et al., U.S. Pat. No. 3,914,573 issued to Muehlberger, U.S. Pat. No. 4,256,779 issued to Sokal et al., U.S. Pat. No. 4,732,311, U.S. Pat. No. 4,841,114 issued to Browning, U.S. Pat. No. 5,298,714 issued to Szente et al., and U.S. Pat. No. 5,637,242 issued to Muehlberger) all use extremely high temperature gases to thermally soften or melt powder particles as the primary consolidation mechanism to achieve practical deposition efficiencies. More importantly, the thermal and plasma spray processes all disperse the thermally soften or melt powder particles over a broad solid-angle cone at large standoff distances that permits air and unwanted gases to be entrained in the spray effluent leading to high levels of oxidation and chemical combustion particularly for reactive metal powders such as aluminum, magnesium, or titanium.
The high velocity methods identified in U.S. Pat. Nos. 2,714,563, 3,914,573, 4,256,779, 4,732,311, 5,637,242, 5,766,693 issued to Rao, and RU Patent 1773072 issued to Alkhimov et al., disclose the advantage of using high velocity particles in addition to thermally softened or melted particle states for enhanced deposition efficiency and improved coating properties.
In contrast, the reexamined coating patent (U.S. Pat. No. 5,302,414B1) issued to Alkhimov et al. restricts the gas-dynamic spraying method to accelerating the gas and particles into a supersonic jet at particles temperatures sufficiently low so as to prevent thermal softening or melting of said particles. Although the thermal softening temperature is not adequately defined in the Alkhimov et al. patent the process is specified to be much below the melting point of the material. Specific examples in the specification indicate that the deposited material does not exceed 100° C. Thus, the Alkhimov et al. patent is limited in its claims in terms of controlling the consolidation physical state of the applied coatings and the process results in coatings with low deposition efficiency and high residual stresses. A more recent U.S. Pat. No. 6,139,913, issued to Van Steenkiste et al. claims improvements to U.S. Pat. No. 5,302,414B1 by including particle sizes in excess of 50 microns. This patent also accelerates gas and particles into a supersonic jet while maintaining the temperature of the gas and particles sufficiently low to prevent thermal softening of the particles. Both of these patents restrict the prior art to applications using supersonic jets.
Plasma spray guns disclosed in U.S. Pat. Nos. 3,914,573, 4,256,779, 4,689,468 issued to Muehlberger, U.S. Pat. Nos. 4,841,114, and 5,637,242 all inject the powder particles into a plasma stream typically at the throat of a nozzle designed to flow a supersonic plasma jet. U.S. Pat. No. 5,298,714 issued to Szente, et al. discloses a plasma torch or gun for deposition of particles onto a substrate in which the particles are injected at the inlet to the nozzle. U.S. Pat. Nos. 3,914,573, 4,841,114, and 5,766,693 specifically disclose methods for thermally softening or eliminating excessive heating of powder particles in a plasma gun, where the particles are heated after expansion of the supersonic plasma stream gas through a converging-diverging nozzle. All of the prior art plasma guns are configured to pass the ionized high-temperature plasma through an outlet or supersonic nozzle prior to deposition on the substrate. This approach precludes in-situ low temperature control of the powder consolidation state in close proximity to the substrate impingement point. In fact, U.S. Pat. No. 4,256,779 requires supplemental cooling of the substrate in order to avoid overheating. Furthermore, the supersonic flow specified in the prior art is very inefficient in terms of accelerating powder particles. This is particularly true once the flow begins the rapid expansion to ambient pressure in the divergent section of a supersonic nozzle. Thus the prior art restricts significant particle acceleration to the short, relatively low velocity, converging section, and the very short throat section of the nozzle. The complexity, inherent in the prior art in plasma guns, increases the cost of these devices for commercial applications. More importantly these conventional plasma guns wastes a large quantities of energy in the form of heat that must be carried away by the cooling water used to keep the electrodes and nozzles from melting or eroding.
Plasma cutting torches (e.g., U.S. Pat. No. 6,002,096 issued to Hoffelner et al.) frequently use a DC transfer-arc to melt or burn (oxidize) a substrate, but this prior art is restricted to cutting applications and does not claim a method for coating, spray forming, joining, or fusing materials using entrained powder particles in the carrier gas. Applications using plasma transfer-arc torches with filler metal powders entrained in the plasma gas are disclosed in U.S. Pat. No. 5,705,786 issued to Solomon et al. and U.S. Pat. No. 6,084,196 issued to Flowers et al. to weld various substrates. U.S. Pat. No. 4,471,034 issued to Romero et al. teaches a method for applying a weld-bonded coating to cast iron parts using a transfer-arc plasma torch. Most of the plasma transfer-arc torches use conventional prior art with a central electrode surrounded by a concentric electrode to generate an arc in the circumferential passageway between the electrodes. U.S. Pat. No. 5,070,228 issued to Siemers et al. generates a plasma plume via a RF coaxial induction coil surrounding the plasma cavity. Powders entrained in the plasma gas or a separate carrier gas (generally argon) are introduced into the arc or plasma to melt the particles. Thus, ionization of the plasma gas occurs internal to the plasma torch or gun with powder particles introduced at low velocities into the plasma stream within the torch or gun housing or adjacent to the plasma stream immediate to the exit orifice.
Plasma heaters and burners have been used to heat and ionized gas (e.g., U.S. Pat. No. 3,601,578 issued to Gebel et al.) and to improve combustion efficiency (e.g., JP 60078205 A issued to Toshiharu), but such devices have not been used to thermally treat particles prior to depositions of coatings. U.S., Pat. No. 5,766,693 discloses a method for applying metal base coatings using a conventional plasma spray gun in which particles are injected into the supersonic jet at temperatures that plasticize the particles, but do not melt the material. External cooling of the substrate is required for this device in order to prevent overheating of the coating and workpiece.
U.S. Pat. Nos. 4,328,257, 4,689,468, 4,877,640 and 5,070,228 issued to Siemers et al. disclose various techniques for electrically coupling a high temperature and plasma stream to the workpiece or substrate using a DC power supply of a given polarity connected between the plasma gun and the target workpiece. These patents teach the use of a high current DC transfer-arc process to preheat the substrate surface, reduce oxide contamination of plasma coatings, or to remove oxide coatings from the metallic particles traveling in the plasma stream. These patents do not teach a method for controlling the deposition and consolidation states of coatings at temperatures below the material melting point. Furthermore, these low-pressure plasma guns or torches have the commercial disadvantage of requiring costly vacuum chambers and equipment to produce the plasma stream.
Thermal softening nomenclature has been used in U.S. Pat. No. 3,914,573 issued to Muehlberger to describe the physical state of powder particles heated to temperatures near the melting point, but below melting. This patent asserts that an optimum particle temperature exists for each specific material. If this temperature is exceeded the particle can spatter upon impact with the workpiece. If the temperature of the particle is too low, insufficient deformation of the particle occurs upon impact resulting in poor quality coatings with poor bonds. The Muehlberger patent further asserts that the addition of thermal energy to the kinetic energy of the particle results in greater deformation of the particles upon impact. Thus the temperature of the particle in combination with the kinetic energy is critical to attain sufficient particle deformation leading to high deposition efficiency, high bond strength, and low porosity.
Two other patents, U.S. Pat. No. 5,766,693 to Rao and U.S. Pat. No. 4,256,779 to Sokol et al. use the term “plasticized” to describe a powder particle temperature state near the melting point of the particle. U.S. Pat. No. 5,766,693 restricts the melted or plasticized state substantially to the surface region of each particle. Sokol, et al. teaches in U.S. Pat. No. 4,256,779 a method for heat-softening or plasticizing powder particles. The powder is injected into a temperature controlled plasma stream to heat-soften or plasticize, but not for a sufficient time to liquefy or vaporize. By inference both of these patents teach a method that is consistent with U.S. Pat. No. 3,914,573 issued to Muehlberger in which the powder particles are heated to temperatures near the melting point.
Other patents teach a broader definition for thermal softening of materials. For example, U.S. Pat. No. 5,312,475 issued to Purnell et al. teach a method for adding submicroscopic carbides to give a resistance to thermal softening of sintered metal materials. This patent reports hardness data for sintered ferrous material that decreases monotonically with increasing temperature of the material from room temperature to 773 Kelvin (500 degrees Celsius). Thus, the thermal softening is demonstrated to have significant effects on mechanical hardness at temperatures significantly below the melting point of iron alloys (i.e., melting point typically in excess of 1500 degrees Celsius).
The objective of the present invention is to overcome the limitations of the prior art by teaching a method for treating the powder particles to alter their physical, chemical, or nuclear properties prior to deposition and consolidation of the solid-state powder particles. The deposition and consolidation process uses a friction-compensated sonic nozzle to accelerate said treated powder particles to high velocity in a subsonic or sonic inert carrier-gas stream in order to apply a coating treatment of an object or to spray form an object. Additionally, the object of the present invention relates to a new method and process for applying various multi-layer coatings, functionally graded materials, functionally formed in-situ composites, and ex-situ composites onto substrates for surface modification and consolidation. The invention also teaches a spray forming method for consolidating powders (metallic, nonmetallic or mixtures thereof) onto a substrate surface while controlling the metallurgical, chemical, or mechanical properties of the substrate and consolidated material. Limitations of conventional thermal and plasma spray techniques are overcome with the present invention by using an inert carrier gas formed into a directed subsonic or sonic jet that significantly reduces oxidation and chemical combustion of nearly molten or molten powder particles (near the melting point of powder particle material) during the deposition and consolidation process. Reduction of oxidation and chemical combustion of the powder particles is achieved because the process reduces mixing and entrainment of air and unwanted gases into the directed jet of inert gas prior to deposition or consolidation on the object at relatively short standoff distances. The invention also provides the means of using a surrounding inert gas shield to further reduce or eliminate entrainment of air or unwanted gases into the directed jet of inert carrier gas. Finally, the invention reduces oxidation and chemical combustion of the powder particles even further by thermal plastically conditioning the powder particles within an inert carrier-gas environment at relatively low temperatures compared to nearly molten (near the melting point of powder particle material) or molten powder particles temperatures used in conventional thermal and plasma spray methods.
Aluminum alloys frequently require coatings for corrosion protection, wear resistance, optical reflectivity, soldering, brazing, welding, machining, and polishing. These coatings must be applied while controlling the metallurgical, chemical or mechanical properties of the substrate and deposited material.
Conventionally, products such as aluminum heat exchangers are manufactured using aluminum braze sheet. The braze sheets is clad with a eutectic outer layer. Aluminum brazing techniques are adequately reviewed in the Aluminum Brazing Handbook [The Aluminum Association, 900 19th Street, NW, Washington, D.C. 4th Edition 1998]. The brazing process consists of wetting the aluminum alloys to be joined with a filler material (e.g., typically 4000 series aluminum-silicon alloys) that enables metallurgical bonding of the joint.
Cladding techniques have been used for modifying the surface of aluminum alloys for many applications, but the process is costly and is primarily amenable to sheet stock. U.S. Pat. No. 3,899,306 issued to Knopp, et al. discloses a method for brazing aluminum parts by applying a thin layer of nickel powder (unconsolidated) between the adjacent surfaces of a pair of parts that are pressed together and heated to a temperature of 537 to 650° C., but below the melting point of said parts. U.S. Pat. No. 3,970,237 issued to Dockus, et al. discloses a method of brazing aluminum parts where clad filler (e.g., aluminum silicon alloy) is plated with a bond-promoting alloy (e.g., nickel-lead or cobalt-lead) between the aluminum parts to enable the brazing process. This patent also teaches the same method of brazing aluminum to braze other materials including steel, aluminized steel, stainless steel, or titanium.
Attempts to use thermal and plasma spray methods for depositing thermally softened or molten braze alloys onto aluminum alloys as disclosed in U.S. Pat. No. 4,732,311 issued to Hasegawa et al. have been largely unsuccessful because of low adhesion (which causes flaking of the coating material during subsequent forming steps). Other factors include 1) oxidation, 2) metallurgical alteration of the substrate induced by undesirable heat treatment, 3) metallurgical alteration of the substrate induced by undesirable diffusion of contaminates, 4) thermal and mechanical distortion of the substrate, and 5) other chemical reactivity.
Flux materials, such as potassium fluoro-aluminate salts (International Patent, WO 00/52228 issued to Kilmer, U.S. Pat. No. 3,951,328 issued to Wallace et al., and U.S. Pat. No. 5,980,650 issued to Belt et al.), are applied to the surface of the eutectic clad as a braze bond-promoting substance that displace the oxide from the surface of the aluminum, lower the filler metal's surface tension, and promote base metal wetting and filler metal flow. These coatings are conventionally applied by spraying a liquid mixture of the potassium fluoro-aluminate salt in water or as a composite powder comprising a potassium fluoro-aluminate salt coated on the surface of the eutectic aluminum-silicon alloy powder [Field, D. J., Krafft, R. G., and Hawksworth, D. K. “Composite Deposition (CD) Technology—A Novel Joining Process for Automotive Heat Exchangers.” Paper 35-Proceedings of T&N Leading through Innovation Symposium, Wurzburg-Indianapolis, Ind., 1995]. In other cases, thin nickel or cobalt coatings have been used as bond-promoting flux coatings as disclosed in U.S. Pat. No. 3,899,306 issued to Knopp, et al. and U.S. Pat. No. 3,970,237 issued to Dockus, et al.
U.S. Pat. No. 5,884,388 issued to Patrick et al. discloses prior art for applying a friction-wear coating to a substrate such as a brake rotor. This patent claim's technique for heating the substrate and machining grooves to enhance bonding of a wire-arc spray formed layer. All of the surface preparation and substrate heating processes unique to U.S. Pat. No. 5,884,388 are required to cope with the oxidation of the substrate and coating deposit which reduces adhesion/cohesion. The extensive surface preparations portend a mechanical bond rather than a metallurgical bond.